The present invention relates to a method for coding facsimiles which are acquired when scanning originals which contain halftone areas. A problem to be solved is to provide an optimum transmission and/or storage of the corresponding informational data.
FIG. 1 shows a section enlarged about seven times from the facsimile of a bicolor halftone image that appeared in a daily newspaper. Visible therein is a rectangular pattern of black dots of different size and shape which is inclined by about 45.degree. relative to the page edges. The black dots bleed into one another in dark regions and are in turn composed of an accumulation of quadratic solid areas which correspond to the facsimile scan spots. In the original, the center-to-center spacing of two halftone screening spots (a) amounts to 0.4 mm and the center-to-center spacing of two scan spots amounts to 0.075 mm.
Color halftone images can be printed in the same fashion when they are monochromatic, i.e. are composed of only a single color with locally variable saturation. In this case, the black dots would have to be replaced by dots of the respective color.
Chromatic halftone images (i.e. pictures containing a plurality of hues) as well as monochromatic halftone images whose color is not available as a printing ink are generally produced by superimposed printing of four patterns of different inclination and raster density in the colors yellow, magenta, cyan, and black. Upon transmission or storage and subsequent reproduction of facsimiles that contain halftone pictures, problems result, particularly a disruption due to moire and an unsatisfacotry data compression which are explained in greater detail in "Proc. of the 6th International Conference on Pattern Recognition, Munich 1982, pp. 489-491, Postl, W.: Halftone Recognition By An Experimental Text And Facsimile Workstation", incorporated herein by reference.
German Patent publications OS No. 2 516 322 and OS 3 024 322, both incorporated herein by reference, disclose methods by means of which areas in a facsimile covered by halftone images are searched by automatic methods and the areas which are found are coded with a so-called image code, whereas a so-called text code is employed for the other areas. The terms "image code" and "text code" mean codes that are particularly suitable for the respective structures.
Specifically disclosed in German OS No. 2 516 322 as an image code is a scanning procedure with reduced resolution--i.e., in other words a transformation of the samples which corresponds to an unsharp optical imaging--whose resolution or lack of sharpness is to be dimensioned such that the halftone raster is suppressed, followed by a coding of the transformed samples by delta pulse code modulation (DPCM) or some other code suitable for coding video signals.
Taken into consideration as a text code in the aforementioned publication are all codes suitable for coding bicolor patterns. Such codes have been specified for facsimile transmission in the publications "Standardization of Group 3 Facsimile Apparatus for Document Transmission T4, CCITT Yellow Book 1981, pages 232-236" and "Draft Recommendation T.b (2nd issue), CCITT Temporary Document No. 28, Geneva 1983", both incorporated herein by reference. They are suitable in identical fashion for coding the facsimiles of characters and of line graphics. They are less suited or altogether unsuited for the facsimile transmission of halftone images.
A favorable compression factor is achieved on the one hand and, on the other hand, moire is avoided (the latter not being expressly pointed out in this publication) with the coding method disclosed in German OS No. 2 516 322 which can be applied both to predefined halftone areas as well as halftone areas that have been automatically located. The method, however, has the disadvantage that details, the expanse of which lies on the order of magnitude of the spacing of the halftone raster spots or therebelow (for instance, thin lines, edges, or small characters printed in the picture), disappear or become unclear even though they were still clealy perceptible with the naked eye in the original facsimile.